Spin based device with low transmission barrier

ABSTRACT

An electron spin-based device includes ferromagnetic layers with different coercivities, such that one of such layers is responsive to a magnetic field and the other is fixed. A value of an impedance in the spin based device for a spin polarized current varies in accordance with a relationship between a first changeable magnetization state and a second non-changeable magnetization state associated with such ferromagnetic layers.

RELATED APPLICATION DATA

The present invention claims priority to and is a continuation of allthe following applications Ser. No. 11/086,603 entitled Method ofOperating a Stacked Spin Based Memory, which is a continuation of Ser.No. 10/776,144 entitled Stacked Spin Memory (now U.S. Pat. No.6,870,761) which in turn is a continuation of and claims priority toSer. No. 10/100,210 filed Mar. 18, 2002 entitled MagnetoelectronicMemory Element With Inductively Coupled Write Wires (now U.S. Pat. No.6,741,494). Application Ser. No. 10/100,210 is a continuation of andclaims priority to an application Ser. No. 09/532,706 filed Mar. 22,2000 titled Magnetoelectronic Memory Element With Isolation Element (nowU.S. Pat. No. 6,388,916). The latter application Ser. No. 09/532,706claims priority to and is in turn a divisional application of Ser. No.08/806,028 filed Feb. 24, 1997 entitled Hybrid Hall Effect Memory Device& Method of Operation, (now U.S. Pat. No. 6,064,083). Ser. No.08/806,028 claims priority to and is a continuation-in-part of Ser. No.08/643,805, filed May 6, 1996 titled Hybrid Hall Effect Device andMethod of Operation, (now U.S. Pat. No. 5,652,445), which in turn claimspriority to and is a continuation-in-part of an application Ser. No.08/493,815, filed Jun. 22, 1995 titled Magnetic Spin Transistor HybridCircuit Element, (now U.S. Pat. No. 5,565,695); and said Ser. No.08/806,028 also claims priority to and is also a continuation-in-part ofboth an application Ser. No. 08/425,884, filed Apr. 21, 1995 titledMagnetic Spin Transistor, Logic Gate & Method of Operation, (now U.S.Pat. No. 5,629,549); and an application Ser. No. 08/643,804 filed May 6,1996 titled Magnetic Spin Injected Field Effect Transistor and Method ofOperation, (now U.S. Pat. No. 5,654,566).

The above applications and materials are expressly incorporated byreference herein.

FIELD OF THE INVENTION

The invention relates generally to spin based electronic devices. Inparticular, the present invention relates to a new magnetic spin devicewhich uses low transmission barriers.

BACKGROUND OF THE INVENTION

Solid state Random Access Memories (RAM), presently based on silicontechnology, are the most important high-speed, reprogrammable memoriesin computer systems today. The content of a memory bit is stored in acircuit element called a cell, fabricated in a silicon IntegratedCircuit (IC) chip. Each cell can exist in one of two stable states torepresent a “0” or a “1.” A word is stored in a particular group ofcells in a memory array and can be identified with, and addressed by, aunique numerical address. Thousands to millions of cells can befabricated in a single Very Large Scale Integrated (VLSI) chip alongwith additional address decoding and read/write control circuits. Giventhe address of any word, the memory content of that word can beretrieved during a memory operation time cycle, typically between 5 and200 nsec. Any given bit can be addressed, written, rewritten, read andreread repeatedly, with fast access, read and write times. The desiredattributes of RAM include high speed, low power dissipation, highpacking density, and inexpensive manufacturing cost. Although thetechnology for address decoding, read/write control and read sensing isfairly standard, the kind of circuit element that is used to compriseeach memory cell can vary widely and is the subject of much research anddevelopment.

Conventional DRAM

The Dynamic Random Access Memory (DRAM) cell is popular because the cellsize can be made small, leading to a high packing density and relativelylow cost. The storage element is a capacitor C and the two stable statescan be, for example, the states with stored charge Q and with storedcharge 0. Every cell is connected to an array of write and read wires,also called “bit” and “word” lines. Since one capacitor linked togetherwith other capacitors in an array can lose its charge to its neighbor,the capacitor of each cell is connected to a transistor in that cell soas to isolate it from the array. When the transistor is on there is alow resistance to a write or read wire so that an applied voltage cancharge the capacitor or a sense circuit can determine the stored charge.When the transistor is “off” there is a high impedance to the write orread wire which isolates the capacitor electrically from any otherelement in the array.

The packing density of DRAM memories has undergone steady improvementfor more than two decades. Early DRAM cells used several capacitors andtransistors, but now the most common designs use a single capacitor andsingle transistor because minimizing the number of elements in a cellallows the size of the cell to be reduced to a minimum. Typically, aMetal Oxide Semiconductor Field Effect Transistor (MOSFET) is fabricatedby standard lithographic processing techniques on a silicon substrate.The oxide that isolates the gate from the channel is highly insulating,so that the metallized gate has a capacitance to the rest of the device.In early designs, e.g. with 3 elements per cell, the gate capacitancewas used as the storage capacitance. Single element cells use a MOSFETand a separate capacitor C. Reading is performed with a sense circuitthat compares the charge (or voltage) of C with the charge (or voltage)of a standard capacitor C in a dummy cell. For an accurate readout, thecharge Q stored on the capacitor must be the order of 1 millionelectrons or more. While FETs have benefited from advances in processingtechniques so that they can be fabricated with dimensions smaller than amicron, capacitors have not benefited from similar gains in technologyand it is still necessary for capacitors to have dimensions of order onemicron or larger in order to hold the necessary charge. Thus, the sizeof DRAM memory cells is not decreasing as rapidly as processingtechnology would allow, and one significant drawback of DRAM is thatreliable cells might never be fabricated on a submicron scale.Furthermore, the necessity of comparing capacitance with that of dummycells requires the fabrication of dummy cells which take up extra spaceon the chip. A second disadvantage, for some cell designs, is that theread process drains whatever charge is stored on the capacitor. Thisphenomenon is known as “destructive readout” because the read processdestroys the state of the cell (capacitor), and a rewrite circuit mustbe provided to rewrite the memory after every read. The rewrite circuittakes up space on the chip and the rewrite process can lengthen theamount of time of the read cycle. A third weakness is that the capacitorplate of any cell is never perfectly isolated. There is always somefinite impedance to ground which permits the charge to leak from thecapacitor, and in this process the memory is also lost. To remedy this,a refresh circuit is used to rewrite the memory constantly, typicallyonce every few milliseconds. The refresh circuit takes up space on thechip, uses Central Processing Unit (CPU) time, lengthens the time of theread cycle, and dissipates extra power [typically 0.1 to 0.2 watts permegabyte of RAM]. The power dissipation is of particular concern formemories that are powered by batteries, such as laptop computers, mobilecommunications equipment, and satellite electronics. DRAM cells derivethe name “dynamic” because the memory is dynamic, i.e. it is constantlybeing refreshed, and memory cells which lose their memory when not beingpowered are called volatile. A fourth problem with DRAM is that aparticle radiation, either from background sources or from contaminantson the chip, can cause a spurious discharge of the capacitor and give afalse reading. This is one of the largest error mechanisms limiting thereliability of DRAM.

More recently, cell circuit elements other than typical oxide dielectriccapacitors have been proposed to serve as nonvolatile memory storageelements. The chief advantage of a Nonvolatile Random Access Memory(NRAM) is that memory is retained even when the array is not beingpowered. Thus, power dissipation is minimized since the array draws zeroquiescent power. Furthermore, refresh circuitry is eliminated, thussaving space on the chip and requiring less time of the CPU. Onecategory of nonvolatile circuit elements utilizes magnetic materials andtheir properties. For example, one kind of nonvolatile memory cell usesa circuit element similar to that of DRAM, but the dielectric materialof the capacitor is ferroelectric, i.e. the capacitance has twodifferent values for two different states of the dielectric which aredetermined by application of a magnetic field. Although this cell isnonvolatile, the dielectric properties of the ferroelectric material arerelatively weak so that the size of the capacitor C must be fairly largeand therefore the cell size is necessarily large. This structure hassimilar limitations to the oxide dielectric capacitors described above:the read process is destructive, the cells are susceptible to a particleradiation. Finally, the ferroelectric material degrades with time sothat the cell is incapable of sustaining an infinite number of read andwrite cycles.

Other kinds of circuit elements for nonvolatile memory cells useferromagnetic materials rather than ferroelectric materials. Twodifferent approaches use magnetoresistive elements as the storageelements of the cell.

MRAM

Magnetoresistive Random Access Memory (MRAM) (described in an article byJames Daughton, “Magnetoresistive Memory Technology,” Thin Solid Films216, 162 (1992) which is incorporated by reference herein) employs anarray of bit and word lines. Each bit line is divided into n storagecells. Each cell is a trilayer composed of a ferromagnetic metal baselayer, a nonmagnetic metal middle layer, and a ferromagnetic metal toplayer. The cell has length l, width w and thickness d. Looking at a cellin cross section across the width, there are two stable magnetizationsstates, each with the magnetization of the two ferromagnetic filmsoriented in opposing directions: clockwise and counterclockwise. Theresistance of each cell, measured with a sense current applied along thelength of the cell, is R₁ when the magnetizations are perpendicular tothe sense current (as is the case for the stable magnetization states)and R₁′ if the magnetizations of the ferromagnetic layers are forced tolie parallel to the sense current. Each cell in the bit line isconnected to the next cell with a conducting strip which has resistanceR_(c). Columns of n word lines cross the m rows of bit lines. Eachnonmagnetic word line crosses the top of a cell in each bit line.

The state of cell (i,j) is written by sending current pulses ofappropriate amplitude through bit line i and word line j, causing themagnetization of the cell to orient either clockwise orcounterclockwise. The contents of the cell are read by first biasingword line j with a large enough current so that the magnetizations ofboth ferromagnetic layers are canted to an orientation that isapproximately 45 degrees away from the axis of the bit line. In thisorientation the resistance of the cell (for a sense current appliedalong the bit line) has a value R₂ that is between R₁ and R₁′. Next, asense current is applied along the bit line, and a voltage is measuredacross the bit line, having a value proportional to (n−1)R₁+R₂+nR_(c).Finally, a read current pulse is applied to the word line, in additionto the original bias current. This current pulse changes themagnetization orientation in a direction more nearly parallel to thesense current if the initial orientation was clockwise, or in adirection more nearly perpendicular to the sense current if the initialorientation was counterclockwise. Thus, the voltage across the bit lineeither increases or decreases when the read pulse is applied. A sensecircuit that measures changes of voltage [a derivative circuit] recordsthe positive or negative change as a “1” or a “0.”

By using a derivative sense technique, MRAM avoids the necessity ofelectrically isolating each cell. However, the signal to be sensed isquite small and the signal to noise ratio is poor. The change inresistance that must be sensed during the read process is a smallfraction of R₁, and this small change must be distinguished from abackground of approximately n(R₁+R₂). In practice, two elements arefabricated for each cell [thus doubling the signal], and the readprocess is repeated several times so that the final readout is taken asan average of repeated samplings [thus lowering the noise]. Thisincreases the time for a read cycle. Power dissipation is relativelylarge during readout because relatively large currents must be appliedto long, resistive lines. Finally, errors can be introduced duringreadout if the bias current tips the magnetization into an unstablestate.

NRAM with Magnetoresistive Element

Another approach uses a magnetoresistor R as the storage element, andthe cell is comprised of R, a reference resistor R′, and means (such asone or more FETs) to isolate the cell from the rest of the array. Themagnetoresistor R is typically a thin film ferromagnetic metal (orferromagnetic/nonmagnetic metal multilayer) resistor with length l,width w and thickness d, and has two values, R′ and R′+δR, correspondingto two stable magnetization states. For example, in one state themagnetization of a permalloy film might be parallel to the direction offlow of the sense current, I_(sense), and in the other state themagnetization might be perpendicular to I_(sense). The magnetizationstate is written by using the magnetic field generated by current pulsesapplied to an array of write wires. The read process begins by selectinga cell, for example by switching the isolating FETs to the “on” state bydriving the appropriate word line to a high voltage. A bias currentI_(sense) is applied to the magnetoresistor R and the reference resistorR′. A sense circuit at the end of a line of cells compares the twovoltages and interprets a “1” or “0” when, for example,I_(sense)*R−R′)>0 or I_(sense)*R−R′)=0 respectively. The voltage levelscorresponding to “1” (and “0”) are then amplified to TTL or CMOS levels.The voltage I_(sense)*δR that distinguishes a “1” from a “0” must belarge enough for reliable discrimination. Since the magnetoresistiveratio δR/R′ of ferromagnetic films (or multilayers) is small, 10 percentor less, the magnetoresistor must be made quite large. For example, withR′=100Ω and δR/R′=0.1, a reasonable bias current of 1 mA would produce adetectable voltage difference of 10 mV.

This approach has several disadvantages. Resistors require substantialarea in a cell. For example, the 100Ω magnetoresistor could befabricated, using ferromagnetic materials with resistivities of about 20μΩ-cm, with a length l=5 μm, width w=1 μm, and thickness d=0.01 μm.Since this cell requires the fabrication of two resistors, substantialspace is used. The reference resistor cannot be placed outside the cellbecause the resistive difference, δR, is so small that the resistance ofeach cell must be matched to a particular reference. Since resistance isa function of temperature, R=R(T), the reference resistor must befabricated very near the magnetoresistor so that both resistors willalways be at the same temperature, and the material for the referenceresistor must be carefully chosen so that the temperature dependence ofits resistivity is similar to that of the magnetoresistor. Finally, theresistance of each cell is quite large. When numerous cells are placedon a single read line, as in an array, the resistance of the read lineis substantial. Since the read process uses current bias, the powerdissipated in each read cycle is relatively large.

Magnetic Spin Transistor

The Magnetic Spin Transistor is described in recent articles (see, e.g.,Mark Johnson, “The All-Metal Spin Transistor,” IEEE Spectrum 31 (5), 47(1994), and Mark Johnson, “The Bipolar Spin Switch,” Science 260, 320(1993), which are incorporated by reference herein) as a novel devicethat can be used as a circuit element in a nonvolatile memory cell. Twocharacteristics of the device must be considered when using the devicein an NRAM array. First, the device can be fabricated entirely frommetals, and is therefore characterized by a low electrical impedance.Thus, to fabricate an array of such elements it is necessary toelectrically isolate each element from others in the array, so that theoutput of any element will not be shorted to ground through aneighboring element. Second, [like many other memory elements] theoutput voltages available from the device are less than TTL or CMOSlevels, and the output must therefore be amplified before it isincorporated in TTL or CMOS circuits.

Previous NRAM cell designs using the spin transistor as a memory elementhave approached the above constraints in two ways. In the first, thememory cell is comprised of a spin transistor and a differentialamplifier. The amplifier also provides cell isolation. The drawback ofthis approach is that each cell requires a significant amount of spaceon a chip, and the complicated fabrication procedures required forcircuits composed of numerous transistors will drive up the cost per bitof the memory. In the second approach, the cell is composed of a spintransistor and one or more capacitors and resistors. The passiveelements provide isolation for the spin transistor of each cell, and thereadout voltage is transmitted to the end of a line of elements whereamplification is performed. The disadvantage here is that passivecircuit elements, in particular resistors and capacitors, requiresubstantial space on a chip. Thus, the majority of cell area is occupiedby passive elements and packing densities are limited. Furthermore, cellisolation is not very efficient and the readout voltage can be degradedduring transmission to the sense circuit, resulting in higher noise andlower readout sensitivity.

SUMMARY OF THE INVENTION

An object of the present invention therefore is to provide a new,nonvolatile, smaller storage element with greater isolation and superiorsignal to noise readout characteristics, for use as the storage cell ina [random access] memory circuit.

Another object of the present invention is to minimize support circuitryon the chip by eliminating dummy memory cells and rewrite and refreshcircuits.

Another object of the present invention is to shorten the time of theread cycle by eliminating the rewrite process.

Another object of the present invention is to increase the efficiency ofa CPU by eliminating the refresh process.

Another object of the present invention is to reduce power consumptionby making the memory nonvolatile, and to reduce the power used during acurrent biased read cycle by minimizing the resistance of the storageelement of each cell.

Another object of the present invention is to minimize memory errorscaused by radiation by using a storage element that is impervious to theeffects of a particles.

According to the present invention, an improved memory cell for NRAM isa hybrid composed of a magnetic spin transistor and a semiconductingisolation element which can be a FET or other similar device. Data canbe read out of the memory cell by applying a select signal to the FET.Similarly, data can be written to the magnetic spin transistor by usinga conductive write layer for carrying a write electric current andinductively coupling a write magnetic field associated with this writecurrent to a ferromagnetic collector layer of the spin transistor.

As with other magnetic spin transistor designs, the ferromagneticemitter and ferromagnetic collector layers of this new transistor areboth fabricated to be magnetically anisotropic so as to permit thecollector to have two stable magnetization states (up and down). Anexternal current generator can change the magnetization state of thecollector by inductively coupling a magnetic field to the collector.Even if power is removed from the above device, the ferromagneticcollector layer magnetization orientation is retained in its set state,thus causing the spin transistor to behave as a non-volatile memoryelement, because the two states of the magnetization orientation of saidsecond ferromagnetic layer can correspond to data values stored in saidmemory element. The magnetic spin transistor can be fabricated on asemiconductor substrate (such as silicon) or on top of an insulating,dielectric layer that coats (and passivates) a portion of asemiconducting device.

The isolation element can be formed from a typical FET that is providedhaving a source, drain and gate with a conducting channel between thesource and drain the conductance of which depends on the voltage appliedto the gate: the conductance is low when no voltage is applied to thegate and high when voltage is applied.

In another embodiment, the hybrid element is used as each cell in anarray of cells. The drain of the FET is attached by a conducting path toa common bit line, the emitter of the magnetic spin transistor isattached to a common read line, and the gate of the FET is attached to acommon word line operated synchronously with the read line.

According to the present invention, the spin transistor is the storageelement, with write and read operations performed in a way similar toprevious spin transistor NRAM cells. Similar to its use in a DRAM cell,the semiconductor transistor is used to isolate the spin transistor fromthe array except when it's being read, and can be called a “cell select”transistor. The cell of the present invention is thus composed of onlytwo elements, and each element can be fabricated at the limits ofprocessing technology; there is nothing known that will prevent therealization of submicron cell dimensions. Amplification can be providedby sense circuits at the ends of the array lines. As the cell size isdecreased, the readout voltage may be degraded by as yet unknownprocessing factors. In this case some amplification can be provided bypreamplifiers associated with blocks of cells.

The improved, hybrid memory cell is superior to the previous spintransistor memory cell because the cell size can be made much smallerand cell isolation is much greater. The hybrid memory cell has numerousadvantages over other kinds of memory cells. The memory element is aspin transistor and therefore a bipolar voltage output can be provided.Thus, the discriminator of the sense circuit can be set to zero (ground)thereby enhancing the ease of readout. Since ground can be made constantat every location on the chip, no dummy cells, reference voltages, norreference resistances are needed.

The hybrid memory cell has specific advantages over DRAM. Memory isnonvolatile, so the cell draws zero quiescent power. The cell can befabricated with smaller dimensions since no passive elements (e.g. nocapacitors) are used. Because readout is nondestructive, refreshcircuitry and dummy cells can be eliminated from the chip. Since spintransistors are made from metals they are not susceptible to a particleradiation, and the memory is radiation hard.

The hybrid memory cell has specific advantages over MRAM. The readoutvoltage is larger, with a superior signal to noise characteristic, sothat read times are much shorter. The bias current is smaller so thatthe power dissipated during the read cycle is much less. The packingdensity is greater because the magnetoresistive element in the MRAM cellis larger and because each MRAM cell requires two elements.

The hybrid memory cell has specific advantages over NRAM designs thatuse magnetoresistive elements. Since the spin transistor can have abipolar output, the discriminator of a sense circuit can be set to zero,which should be constant everywhere on the chip, and no referenceresistance is needed. The cell has fewer elements and can be made muchsmaller, resulting in larger packing densities. Whereas amagnetoresistive element must be fabricated on a large size scale sothat its resistance is large enough [and therefore δR is large enough]to produce a reliably detected voltage, the spin transistor scalesinversely with sample size. In other words, the transimpedance of a spintransistor becomes larger as the dimensions of the device are madesmaller. A spin transistor with interfacial area of 1 μm by 1 μm has atransimpedance of 10Ω (identical with δR of the larger magnetoresistor)but has a parasitic resistance (the resistance which dissipates powerwhen current biased) of only a few ohms. Thus, an array of hybrid spintransistors dissipates less power during the read operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a trilayer structure illustratingthe basic structure and operation of a typical magnetic spin transistor.

FIG. 2 is a cross-sectional view of the trilayer structure in FIG. 1showing an embodiment of an improved magnetic spin transistor whereincircuit elements are used to offset the output current or voltage.

FIG. 3 is a cross-sectional view of the trilayer structure of animproved magnetic spin transistor invention which is usable for anynumber of operating environments.

FIG. 3A is a perspective view of same trilayer structure of an improvedmagnetic spin transistor, showing the paramagnetic base, ferromagneticemitter, ferromagnetic collector and connections thereto.

FIG. 3B is a perspective view of a write wire inductively coupled to animproved spin transistor.

FIG. 3C is a perspective view of two write wires inductively coupled toone spin transistor element in an array of spin transistor elements.

FIG. 4 is a schematic view of a first embodiment of a hybrid spintransistor memory cell comprised of a single magnetic spin transistorand a single select transistor.

FIG. 5 is a schematic view of a second embodiment of a hybrid spintransistor memory cell comprised of a single magnetic spin transistorand two select transistors.

FIG. 6 is a schematic view of a third embodiment of a hybrid spintransistor memory cell comprised of a single magnetic spin transistorand two select transistors, where the reference voltage differs fromthat of FIG. 5.

FIG. 7 is a random access memory circuit schematic depicting the use ofhybrid cells including magnetic spin transistor and FETs in a memoryarray.

DETAILED DESCRIPTION OF THE INVENTION

Before explaining the details of the new hybrid spin transistor memorycell, a brief review of the operating principles and devicecharacteristics of the spin transistor will be provided for purposes ofputting the present invention in context. While specific details of thephysics of these devices is not important for purposes of the presentinvention, a more detailed discussion can be found in the aforementionedScience and IEEE Spectrum articles.

Most embodiments rely on a trilayer structure drawn schematically incross-section in FIG. 1. Ferromagnetic emitter 12 and ferromagneticcollector 14 are thin films of iron, cobalt or permalloy,Ni_(x)Fe_(1-x), although any electrically conductive, single domainferromagnetic layers may be used. Paramagnetic base 16 is gold, copper,or aluminum, but any conductive paramagnetic material, i.e, any materialhaving electron levels that are not significantly affected by theelectron spin so that there is little splitting between the spinsubbands, is acceptable. The equilibrium energy levels for typicalparamagnetic materials are substantially the same for the two electronicspins. Alternatively, a non-paramagnetic material that can be madeparamagnetic through known means is also acceptable.

It will be understood by skilled artisans that the terms “ferromagneticemitter,” “paramagnetic base” and “ferromagnetic collector” serve asshort hand descriptive aids in this field for understanding the presentinvention. In some ways, these terms suggest a natural analogy withsemiconductor devices. While such analogies may be helpful pedagogicalnotions, no limitations or relationships typically associated with orbetween such structures in the physics of semiconductor transistorsshould be presumed to apply to the corresponding ferromagnetic andparamagnetic regions of the present invention. A full understanding ofthe limitations and relationships of such regions in the presentinvention is provided herein, in the above articles, and in other priorart materials discussing magnetic spin transistors.

Each of the three films typically has a thickness greater than anelectron mean free path, typically tens to hundreds of nanometers. Thethickness of the paramagnetic base 16 is preferably less than the spindiffusion length δ_(s) typically of order 1 micron in polycrystallinemetal films such as gold, copper, silver, aluminum or niobium, anddefined to be the length l in the paramagnetic base over which aninduced nonequilibrium magnetization M_diffuses while the amplitudediminishes to 1/e of the initial value, M_=M_(—0) e^(−1/δ).

In general the orientation of the magnetizations of the ferromagneticemitter 12 and ferromagnetic collector 14, M_(E) and M_(C), can lie inany direction. It is common to use ferromagnetic materials with in-planeanisotropies so that M_(E) and M_(C) are restricted to lie in the planeof the films, in which case the device has a continuum of states witheach state corresponding to the projection of the magnetization of M_(C)on M_(E). For digital storage applications, the device is preferablyfabricated using ferromagnetic films with parallel anisotropy axes inthe film plane. The ferromagnetic emitter 12 is fabricated from amaterial with a large coercivity [and/or a large exchange bias orpinning anisotropy] and the ferromagnetic collector 14 is fabricatedfrom a material with a smaller coercivity. Thus, M_(E) is initiallypolled in the up orientation, denoted in FIG. 1 by the up arrow withinthe ferromagnetic emitter 12, and it remains in this state. Under theseconditions a spin transistor acts as a two state device, correspondingto the two stable states of M_(C), up and down (parallel or antiparallelto M_(E)). These two equally likely states are represented in FIG. 1 bythe dashed arrows, up and down, within the ferromagnetic collector 14.For the chosen convention of M_(E) up, the two device states aredetermined by the state of M_(C) and are hereafter labeled as “up” and“down.” Those skilled in the art will appreciate that M_(E) could bechosen down and a two state device would exist with opposite outputpolarity; moreover, an equivalent notation, sometimes used in theliterature, is M_(E) and M_(C) parallel or antiparallel.

A conventional spin transistor such as shown in FIG. 1 is a threeterminal, current biased, low impedance device with a bipolar voltage orcurrent output that depends on the state of the device [i.e. thatdepends on the projection of M_(C) on M_(E)]. An electrical source 18pumps positive bias current I_(E) from the ferromagnetic emitter 12 tothe paramagnetic base 16 and creates a nonequilibrium population of spinpolarized electrons, equivalently a nonequilibrium magnetization M_inthe paramagnetic base. In the simplest analysis, much of the biascurrent returns to the source by the path through node II. Thenonequilibrium magnetization in the paramagnetic base generates anelectric field at the paramagnetic base—ferromagnetic collectorinterface, and the sign of the field depends on the magnetizationorientation of the ferromagnetic collector M_(C) with respect to theorientation of the polarized electrons, and therefore with respect toM_(E). When M_(E) and M_(C) are parallel the electric field generated atthe interface pushes electric current from the paramagnetic base intothe ferromagnetic collector, and when they are antiparallel the fieldpulls electric current from the ferromagnetic collector into theparamagnetic base. Since current in the circuit of FIG. 1 is conserved,the interfacial electric field can be considered as a “battery” thatgenerates a circulating current in the ferromagnetic collector arm ofthe circuit, either clockwise or counter-clockwise. Quantitatively, theinterfacial electric field that is generated by the nonequilibriumpopulation of polarized spins can be described by a transimpedanceR_(S). The spin-coupled voltage V_(S) developed across the interface islinearly proportional to bias current, R_(S)=|V_(S)|/I_(E), where R_(S)is defined as positive and V_(S) is bipolar. The magnitude of R_(S) isinversely proportional to the volume of the paramagnetic conductingmaterial (the paramagnetic base) between the ferromagnetic emitter andferromagnetic collector, and can be of the order of ohms for devicesfabricated with a spatial scale on the order of a micron.

In FIG. 1, the ferromagnetic collector arm of the circuit contains anarbitrary (selectable) load resistance 20, also called R_(L). Theresponse of the spin transistor to several loading configurations cannow be discussed.

In the first extreme case let R_(L)

0 so that the ferromagnetic collector arm of the circuit behaves as ashort circuit ammeter. Then current flow in the ferromagnetic collectorarm will be clockwise and positive, from node I through R_(L) to node toII, when M_(C) is up [M_(E) and M_(C) parallel], and counter-clockwiseand negative (current will flow from II to I) when M_(C) is down [M_(E)and M_(C) antiparallel].

In the second extreme case let R_(L)

∞ so that the ferromagnetic collector arm of the circuit represents anopen circuit voltmeter. Then the voltage V_(I) at node I is positivewith respect to the voltage at the paramagnetic base, V_(I)>V_(II), whenM_(C) is up, and V_(I) is negative with respect to the paramagnetic basewhen M_(C) is down.

Magnetic Spin Transistor With Adjustable Output Offset

A first embodiment of my improved spin transistor design is shownelectrically in FIG. 2. This embodiment shows changes that I havediscovered more realistically model spin transistor devices as realcircuit elements. First, separate grounds have been drawn for theelectrical source 18, paramagnetic base 16 and ferromagnetic collector14 to represent the fact that these components may be grounded atspatially remote parts of a circuit. Second, a parasitic resistance 22,also denoted R_(B) has been explicitly included between the paramagneticbase and ground. This represents the fact that there is always somefinite resistance from the paramagnetic base to ground including, forexample, the intrinsic resistance of the paramagnetic conductingmaterial comprising the paramagnetic base itself. In typical prior artspin transistor embodiments, R_(B) is considered to be negligibly smallor exceedingly large, and in these embodiments the output current orvoltage is limited to be symmetrically bipolar.

In the present invention, R_(B) is adjusted relative to thetransimpedance R_(S) to permit the output of a spin transistor to beoffset by any desired amount to effectuate anything from a full bipolarto a unipolar output for any known load resistance R_(L). The adjustmentof R_(B) relative to the transimpedance R_(S) can be accomplished byaltering the geometry of the paramagnetic base 16 relative to theferromagnetic emitter 12 and ferromagnetic collector regions 14, such asby varying the thickness and transverse dimensions of the paramagneticconducting material outside the region between ferromagnetic emitter andferromagnetic collector, or introducing a low transmission barrier ateither interface or at any boundary to the paramagnetic base region (theregion between ferromagnetic emitter and ferromagnetic collector), or byusing different materials for the paramagnetic base such as niobium. Inthis manner, R_(B) can be made to be on the same order, or larger thanthe transimpedance R_(S).

Magnetic Spin Transistor With Write Wire

The operation of an improved spin transistor invention 10 is shown inFIG. 3. In this figure, while the spin transistor also includesadjustable offset, the parasitic paramagnetic base resistance R_(B) hasnot been included in the drawing and any load resistance has beenreplaced by a meter [detector] 32 that displays the output current orvoltage of the ferromagnetic collector arm of the circuit and hascharacteristic impedance R_(L). A bias current can be applied in theform of digital pulses 40; this can also be considered as a read currentI_(R). The orientation M_(C) of the magnetization of the ferromagneticcollector can be set to a stable state by using localized pulses ofmagnetic field. An integrated wire fabricated in close proximity to theferromagnetic collector film 14 is called a write wire 36. A pulse ofpositive electric current 34, called a write pulse, transmitted down thewrite wire generates a magnetic field 38 close to the write wire. Thewrite wire is situated so that a positive current generates a field 38at the ferromagnetic collector that is positive (up in FIG. 3) and themagnetization orientation of M_(C) will be set upwards in response tothis field if it is of sufficient magnitude. When no current istransmitted in the write wire there is no magnetic field and themagnetization orientation M_(C) retains its initial orientation becauseof hysteresis. If a sufficiently large negative current pulse istransmitted down the write wire, the associated magnetic field pulsewill be negative, pointing downwards at the ferromagnetic collector, andM_(C) will be set to point down. While element 36 in FIG. 3 has beendescribed as a “wire” it will be understood by persons skilled in theart that any number of well-known structures capable of carryingsufficient current (including for example a conductive film, or aninterconnect line) to generate field 38 will be suitable in the presentinvention.

The structural configuration of the preferred embodiment of the improvedspin transistor shown with an integrated write wire is depicted in FIG.3B. Ferromagnetic collector electrode 350 is a bilayer composed offerromagnetic collector 352 and a thin overlayer 354 which serves toimprove current flow and to protect the ferromagnetic collector againstoxidation. The ferromagnetic collector electrode 350 may be rectangularin shape and may have transverse dimensions ranging from 0.1 to 50microns. An electrically insulating material 384 coats a portion of theelectrode. Write wire 382 (typically a metal film or other stronglyconducting film) is fabricated over the insulator 384 so that it iselectrically isolated from the ferromagnetic collector. It is understoodthat this spatial orientation can be inverted with the write wireunderneath; in some geometries the spin transistor element can carrysome of the current of the write pulse.

In FIG. 3B only a portion of the write wire 382 is shown; the wireextends to contact a bipolar current source at one end and a groundwhich is preferably a ground isolated from the magnetic transistorground on the other end. Alternatively, write wire 382 can be connectedto a single polarity source (such as a data input source that variesfrom 0 to some positive value). The vector magnetic field generated bycurrent flow in the write wire 382 points in a circulating direction386. For positive current the field at the position of the ferromagneticcollector 352 is positive along the z axis. The linear relationshipbetween the magnitude of the field at the ferromagnetic collector 352and the magnitude of the current in the write wire 382 is described bythe inductive coupling parameter α. The field magnitude is directlyproportional to the current magnitude, H=αI. As is well known in theart, a depends on the detailed geometry of the write wire 382,ferromagnetic collector electrode 350, and their spatial relationship.As such, it can be selected by a skilled designer to have any desiredvalue. It is understood, for example, that α decreases as the thicknessof the insulating layer 384 increases. In the preferred embodimentsshown herein, α is chosen to have a value between 5 and 20 (in practicalunits where I is in amps and H in tesla).

The amplitude of the write pulse is determined so that the amplitude ofthe local magnetic field at the ferromagnetic collector is greater than(sufficient to overcome) the coercivity of the ferromagnetic collectorand thus set the ferromagnetic collector to a different magnetizationstate. Again, it is well known in the art that the amplitude of thelocal magnetic field amplitude impressed on the ferromagnetic collectordepends on the value of the inductive coupling parameter α, and theamplitude of the write current pulse. In the preferred embodiment, thewrite pulse has a current amplitude of 0.1 mA, and the amplitude of thelocal magnetic field is about 10 Oersted (α=8). The coercivity of theferromagnetic emitter and ferromagnetic collector is selected to be 40and 8 Oersteds respectively. The choice of specific current amplitude,field strength and coercivities to be used can be easily determined byone skilled in the art depending on the specific application desired.

In the detailed preferred embodiment shown in FIG. 3A, the ferromagneticemitter 345 is typically a bilayer composed of a ferromagnetic conductor346 made of iron, permalloy or cobalt (with a thickness 0.06 micron, alength of about 2 microns and a width of 1 micron) fabricated on anonmagnetic conductor 347 which is used either to promote a magneticanisotropy in the ferromagnetic conductor 346 (in which case thematerial could be nickel oxide with a thickness of 0.01 micron) or topromote isotropic current flow into the ferromagnetic emitter 345 (inwhich case the material would be gold, silver, aluminum or copper with athickness of 0.08 micron). The ferromagnetic collector 350 is a bilayercomposed of ferromagnetic conducting material 352 with a thickness 0.06micron, a length of about 2 microns and a width of about 1 micron.Overlayer 354 is made of gold, silver, aluminum or copper, has athickness of 0.08 micron, a length of 2 microns and a width of 1 micron.Write wire 382 is made of gold, silver, aluminum or copper, has athickness of 0.1 micron and a width of 1 micron (the length extends outof the figure). Insulating layer 384 is made of polyimide, aluminumoxide, silicon dioxide or silicon monoxide, has a thickness of 0.05micron, a width of 1.2 microns and a length of 1.5 microns. Theparamagnetic base 335 is made of gold, silver, copper or aluminum with athickness of 0.1 micron, a width of 1 micron and a length of 2 microns.

As will be understood by those skilled in the art, the materials anddimensions described for the above structures are not critical withinmost reasonable limits. Typically, there are wide ranges of acceptablevalues for any particular application, and the final choice can be madeon the operating requirements of any chosen application for suchmagnetic spin transistors.

An alternative embodiment of the present invention, depicting theimproved magnetic spin transistor is shown in FIG. 3C. In suchapplications, an array of write wires is used. In FIG. 3C ferromagneticcollector electrode 350 of each element of the array is fabricated inthe vicinity of a pair of write wires, and the pair is unique for eachelement. Write wire 356, a segment of which is shown, is one member [I]of a column of [n] write wires, and write wire 358 is one member [j] ofa row of [m] wires, with all n+m wires used to address each of the n*melements of the [n] by [m] array. Each write wire 356 and 358 isconnected to a bipolar current source at one end and ground at the otherend, and the two wires are electrically isolated from each other byinsulating layer 368 and from the ferromagnetic collector electrode byinsulating layer 364.

The magnetic field at each ferromagnetic collector 352 of the array isthe sum of the fields 362 and 360 generated from current in each wire356 and 358. The current amplitude for the pulses simultaneously appliedto each line and the inductive coupling parameter for each line areadjusted so that the net field H at each ferromagnetic collector 352 isslightly larger than the coercivity of the ferromagnetic collector.However, the field generated by either write line alone is less than thecoercivity. Thus, appropriate current pulses of positive or negativepolarity transmitted down the [I] and [j] write lines will orient themagnetization of the ferromagnetic collector of the element at the sitewith the address (i,j) to be positive or negative (up or down) but themagnetization orientation of the ferromagnetic collectors at other sitesin row [I] or column [j] will not be affected.

First Embodiment of Hybrid Memory Cell with Magnetic Spin Transistor andAssociated FET

A first embodiment of a spin transistor hybrid memory cell 400 as it maybe used in a memory array is shown schematically in FIG. 4. Write wiresare not shown in the figure and write functions are performed in thesame manner as described above. A description of the read operation nowfollows. The cell 400 is denoted by the dotted line surrounding the twocircuit elements: a spin transistor 10 is the storage element and a FET420 [a bjt could also be used] is hereafter referred to as a selecttransistor. A read pulse I_(R) applied to the input terminal 402provides the bias current for reading the state of the spin transistor,e.g. a positive output voltage [with respect to ground] representing a“1” or zero [or negative] voltage representing a “0” is developed at thecollector 14 and delivered to the FET source 406. The enhancement modeFET 420 performs a cell select function in the same way as done in atypical conventional semiconductor memory array. Gate 404 is biased atterminal 410 by a voltage V_(D) which controls the resistance of thesource—drain channel beneath gate 404. Normally the gate 404 is notbiased, and since the resistance of the source—drain [406-408] channelin this state is very large [order of many MΩ or GΩ] the spin transistor10 is effectively isolated from the common bit line at terminal 424 andthus from other elements in the array.

When the contents of the cell are to be read, a current pulse, appliedthrough a read [or bias] line and a voltage pulse, applied through aword line are simultaneously applied to the input terminal 402 of thespin transistor and the select terminal 410 of the FET. The source—drainresistance is now low [the range spans the order of 1000 ohms down toless than an ohm] and the output voltage, V_(O), developed at thecollector 14 of the spin transistor is transmitted to the outputterminal 424. It is understood that the [terminals] (402, 410, 424) ofeach cell 400 are connected to wires that are common to all cells in thearray. In particular, the bit line that connects to terminal 424transmits V_(O) to a high input impedance sense amplifier [orpreamplifier] (not shown) at the end of the line as done in otherconventional semiconductor memory designs. In designing the detailedcharacteristics of cell 400, the important criterion (when using avoltage sense amplifier) is that the resistance of the select transistormust be much less than the input impedance of the sense amplifier. Inthis way only a small fraction of the output voltage is dropped acrossthe select transistor and nearly the full voltage magnitude is availableat the sense amplifier input. Since the impedance of the latter can beof order of many MΩ or [even GΩ], the select transistor can be designedusing well known and conventional techniques to have an “on” resistanceof order 1Ω up to the order of 10 KΩ. It should be understood that theschematic drawing of FIG. 4, is not drawn to scale, and in practice thespin transistor may be made much smaller that select FET 402. Inparticular, spin transistor 10 can be fabricated on a scale withdimensions of 1 micron or less, and can preferably, to reduce circuitarea, be fabricated on an insulating layer on top of the selecttransistor. Thus, the hybrid cell can be reduced to size limits imposedonly by existing lithographic processing technology.

Second Embodiment of Hybrid Memory Cell

A second embodiment of a spin transistor hybrid memory cell 500 is shownschematically in FIG. 5. This embodiment is especially suitable fordesigns where the [electronic] ground plane of the chip is not constantover a large area and, it is therefore advantageous to provide a voltagereference from each cell. In addition, it might be desired to connectthe common read line to a column of magnetic spin transistors in seriesrather than in parallel [i.e. connect the ground of one cell to theinput of the next]. In this case it would be necessary to provide areference [ground] for each cell.

When the ground plane is not constant it may be difficult to utilize abipolar output. The bipolar output of the spin transistor can be easilyoffset, however, by manipulating parameters such as R_(B) and thegeometry of the device. For example, it is convenient to offset theoutput by choosing R_(B)=R_(S) so that the voltage output is 2*I_(R)*R_(S) for a “1” and 0 for a “0.” In this case a reference voltage V_(R)for each cell can be provided at the base 16 of the spin transistor 10.As before, a select transistor 520 isolates the collector 14 from thebit line. Here a second select transistor 540 isolates the base from thebit reference line, where the bit and bit reference lines are commonlines for the array and are connected to a sense amplifier, describedbelow. The read operation proceeds in a manner that is similar to thatdescribed above. A current pulse [typically of order 1-10 mA], appliedthrough the read [or bias] line, and a voltage pulse [of order 1-5 V],applied through the word line, are simultaneously applied to the inputterminal 502 of the spin transistor and the select terminals 510 and 512of the two select transistors. The source—drain resistance of eachtransistor is now low and the output voltage V_(O) developed at thecollector 14 of the spin transistor and the reference voltage V_(R) fromthe base 16 of the spin transistor are available at the output terminal524 and reference terminal 526. A bit line and a bit reference line,common to the elements in a column of the array, transmit V_(O) andV_(R), respectively, to a high input impedance sense amplifier [orpreamplifier] at the end of the line.

Third Embodiment of Hybrid Memory Cell with Bipolar Output andConfigurable Offset

A third embodiment of a spin transistor hybrid memory cell 600,conceptually very similar to the second, is presented in FIG. 6. If theground plane is not constant over a large area of the chip but the useof a bipolar sense circuit is still desired, this third embodiment canbe used. Here the reference voltage V_(R) is taken from the base, at alocation of the base that is remote from the base—collector interfaceand which is independent of the base resistance R_(B). This figuredemonstrates that a variation of sample geometry, as well as selectionof an appropriate value for R_(B), can be used to configure the offsetvoltage of the spin transistor. In the configuration schematically inFIG. 6, the offset [relative to the reference] is zero regardless of thevalue of R_(B), and the output voltage, with respect to this reference,is symmetrically bipolar. As before, a select transistor 620 isolatesthe collector 14 from the bit line and a second select transistor 640isolates the base from the bit reference line. The bit and bit referencelines are common lines for the array and are connected to a senseamplifier. The read operation proceeds in a manner that is similar tothat described above. A current pulse [typically 1-10 mA], appliedthrough the read [or bias] line, and a voltage pulse [typically on theorder of 1-5 V], applied through the word line, are simultaneouslyapplied to the input terminal 602 of the spin transistor and the selectterminals 610 and 612 of the two select transistors. The source—drainresistance of each transistors now low and the output voltage V_(O)developed at the collector 14 of the spin transistor and the referencevoltage V_(R) from the base 16 of the spin transistor are available atthe output terminal 624 and reference terminal 626. A bit line and a bitreference line, common to the elements in a column of the array,transmit V_(O) and V_(R) respectively, to a high input impedance senseamplifier [or preamplifier] at the end of the line.

Hybrid Memory Cell Array

The integration of a magnetic spin transistor hybrid memory cell as eachelement in a random access array of elements is depicted schematicallyin FIG. 7 for the example of a 2×4 cell array. This example uses thesecond embodiment of a hybrid magnetic spin transistor, depicted indetail in FIG. 5, but it is understood that a similar diagram could bedrawn with any of the embodiments forming the contents of the cell, withappropriate adjustments made for the sense amplifiers and bit lines. Toread the contents of cell (C1, R1) 702, for example, a voltage isapplied to the word line C1 to select all cells in column 1. Inparticular, the select transistors 704 and 706 of cell 702 are driven tothe “on” state and the paths from the collector 708 and base 710 of cell702 to the sense amplifier 716 are now conductive. A bias currentapplied to read (bias) line B1 develops a voltage at the collector 708relative to the base 710, and the voltage is transmitted down bit line718 and bit reference line 720 to the input 722 and reference 724terminals of the sense circuit. The voltage is amplified and is madeavailable at the output terminal 726. The sense circuit may be the soleamplifier on the line, providing, for example, a TTL or CMOS compatiblevoltage at the output 726, or it may be a preamplifier that isassociated with a block of cells and transmits a voltage to a line R1that is common to other blocks of cells and that delivers the voltage toa secondary sense circuit at the end of line R1. All cells in column C1are read during this process. The contents of the particular cell 702are identified by addressing the output 726 of the sense circuit of rowR1.

Because the bit and bit reference lines are common to all cells 702,730, 740, and 750 in row R1, the importance of the isolation transistorsis readily apparent. If, for example, the isolation transistors 732 and734 in cell 730 were absent, a high conducting path [through the lowimpedance spin transistor 736] would connect the bit and bit referenceline and the readout voltage from cell 702 would be destroyed by theshort circuit.

In the schematic drawing of FIG. 7 the cells on each read line areconnected in parallel. Each spin transistor has an electrical impedanceof order 1 ohm and it is understood that the resistance of the bias lineis small compared to that of each spin transistor. It is also understoodthat the cells could equally well be connected in series. In any case,the read line can be matched to a standard 50Ω source and the powerdissipated during the read cycle [using about 1 mA for a bias current]is of order 50 microwatts. The power per cell is minimal, and localheating of the cell is minimal.

The diagram of FIG. 7 has been drawn for circuits operating with abipolar power supply, typically −5 to +5 volts. It is often desired tooperate a circuit with a single power supply, e.g. 0 to +V volts. Itwill be clear to persons skilled in the art that the circuit of FIG. 7functions in the same manner when ground is biased up to 2.5 V usingstandard techniques. Then “ground” has the usual meaning, as themidpoint of the circuit, but is really at (e.g.) 2.5 V rather than 0 V.

The discussion above has been based on the use of FETs as part of thehybrid cell, but in principle other isolating (select) elements shouldalso work equally effective. For example, bipolar junction transistors,thin film transistors, etc., should work in the same manner.

It's also possible to fabricate multiple layers of spin transistors on achip, in a stack arrangement. A first array of cells could be made usingthe method described above, followed by a planarization layer (e.g.,such as polyimide). The polyimide layer passivates and acts as aninsulator. Then additional layers of spin transistors could be formedabove this first layer, device etc. In this way density would beincreased by a factor of n where n is the number of layers in the stack.

One problem with this type of stack arrangements is that the isolationtransistor can be readily fabricated only on the first layer, i.e. onthe surface of the Si substrate. However, other kinds of select devices,such as a thin film FETs, amorphous silicon FETs, etc. could also beformed in intermediate layers to form alternating layers of selecttransistors and magnetic spin transistor storage elements.

Alternatively, if a single select FET is fabricated on the substrate,several spin transistors, in a stack, could be connected to the oneselect FET on the bottom (on the Si surface), thus allowing all of thespin transistor states to be read out at once. For example, if the stackhad 8 bits, then it is possible to address and read out a byte at atime. One scheme for reading out several bits at a time in this mannerwould involve giving each magnetic spin transistor in a given layer atransimpedance unique to that layer so that the output voltage could bedeconvolved into the bits for the cell of each layer.

Although the present invention has been described in terms of apreferred embodiment, it will be apparent to those skilled in the artthat many alterations and modifications may be made to such embodimentswithout departing from the teachings of the present invention. Forexample, it is apparent that other types of memory cell elements beyondthose illustrated in the foregoing detailed description can be formedusing the teachings of the present invention. Accordingly, it isintended that the all such alterations and modifications be includedwithin the scope and spirit of the invention as defined by the followingclaims.

1. A spin-device including a thin film stack comprising: i) a firstferromagnetic layer characterized by a first coercivity and a firstmagnetization that is responsive to a switching magnetic field imposedon the spin-based device; ii) a second ferromagnetic layer characterizedby a second magnetization and a second coercivity which is substantiallygreater than said first coercivity, such that said second magnetizationdoes not change in response to said switching magnetic field; wherein animpedance of the spin-based device to a spin based current varies inaccordance with a magnetization orientation relationship between saidfirst ferromagnetic layer and said second ferromagnetic layer; iii) athird layer including at least one of gold, silver, aluminum and/orcopper situated between said first ferromagnetic layer and said secondferromagnetic layer; iv) a fourth layer located in said thin film stackwhich is adapted to promote isotropic flow for said spin based current.2. The spin-based device of claim 1, wherein at least one of said firstferromagnetic layer and said second ferromagnetic layer comprises amulti-layer of two different materials.
 3. The spin-based device ofclaim 1, wherein the device is characterized by a transimpedance valuewhich increases as dimensions of said device decrease.
 4. The spin-baseddevice of claim 1, wherein said fourth layer includes at least one ofgold, silver, aluminum and/or copper.
 5. The spin based device of claim1, further including a low transmission barrier layer situated betweensaid first ferromagnetic layer and said second ferromagnetic layer. 6.The spin based device of claim 5, wherein said low transmission barrieris at an interface or a boundary of said third layer.
 7. The spin baseddevice of claim 1 wherein said third layer is a paramagnetic material.8. The spin based device of claim 1 wherein said third layer includescopper and is less than approximately 1 micron in thickness.
 9. The spinbased device of claim 1 wherein said first coercivity is less than halfof said second coercivity.
 10. The spin based device of claim 1 furtherincluding a fifth layer including at least one of gold, silver, aluminumand/or copper situated near said first ferromagnetic layer and which isadapted to improve current flow and protect against oxidation of suchferromagnetic layer.
 11. The spin based device of claim 1, wherein saiddevice is situated on a silicon substrate.
 12. The spin based device ofclaim 1, wherein said spin based current flows perpendicular to amagnetization plane of said first ferromagnetic layer and said secondferromagnetic layer.
 13. A spin-based device including a thin film stackcomprising: i) a first ferromagnetic layer characterized by a firstcoercivity; ii) a second ferromagnetic layer characterized by a secondcoercivity which is substantially greater than said first coercivity;iii) a third layer including at least one of gold, silver, aluminumand/or copper situated between said first ferromagnetic layer and saidsecond ferromagnetic layer; iv) a fourth layer located in the thin filmstack which is adapted as a low transmission barrier to a spin basedcurrent.
 14. The spin-based device of claim 13, wherein said firstcoercivity is less than half of said second coercivity.
 15. Thespin-based device of claim 14, wherein said first coercivity is lessthan 10 oersteds.
 16. The spin-based device of claim 13 wherein anamplitude of a switching field used to change a first magnetization ofsaid first ferromagnetic layer is about 10 oersteds, and said secondferromagnetic layer has a second magnetization which does not change inresponse to said switching field.
 17. The spin-based device of claim 13,further including a fifth layer located in the thin film stack adaptedto promote magnetic anisotropy in said second ferromagnetic layer. 18.The spin-based device of claim 13, further including a fifth layerlocated in said thin film stack adapted to promote isotropic currentflow.
 19. The spin based device of claim 15, further including a nickeloxide layer situated on said second ferromagnetic layer.
 20. The spinbased device of claim 13, wherein said low transmission barrier is at aninterface or a boundary of said third layer.
 21. The spin based deviceof claim 13, further including a fifth layer located in the thin filmstack which is adapted as a second low transmission barrier to a spinbased current.
 22. The spin based device of claim 13 further including afifth layer layer including at least one of gold, silver, aluminumand/or copper which is situated on said first ferromagnetic layer. 23.The spin based device of claim 22, wherein said fifth layer is adaptedto improve current flow and protect against oxidation of suchferromagnetic layer.
 24. The spin based device of claim 13, wherein saidspin based current flows perpendicular to a magnetization plane of saidfirst ferromagnetic layer and said second ferromagnetic layer.
 25. Thespin based device of claim 13, wherein said device is situated on aninsulator
 26. The spin based device of claim 25, wherein said insulatoris situated on a silicon substrate.
 27. The spin based device of claim26, further including one or more semiconducting devices in said siliconsubstrate.
 28. A spin-based device configured as a thin film stackcomprising: i) a first ferromagnetic layer with a first variablemagnetization state which can be changed in response to a magneticfield; ii) a second layer including at least one of gold, silver,aluminum and/or copper which is adapted for preventing oxidation andenhancing current flow in said first ferromagnetic layer; iii) a secondferromagnetic layer with a second magnetization state which is fixed;iv) a fourth layer including at least one of gold, silver, aluminumand/or copper situated between said first ferromagnetic layer and saidsecond ferromagnetic layer; v) a fifth layer in the thin film stackwhich is adapted as a low transmission barrier to a spin based current.29. The spin based device of claim 28, wherein said device is situatedon an insulator.
 30. The spin based device of claim 29, wherein saidinsulator is situated on a silicon substrate.
 31. The spin based deviceof claim 30, further including one or more semiconducting devices insaid silicon substrate.
 32. The spin based device of claim 28, furtherincluding a sixth layer adapted to promote isotropic current flow insaid second ferromagnetic layer.
 33. The spin based device of claim 28,further including a sixth layer adapted to promote magnetic anisotropyin said second ferromagnetic layer.
 34. The spin based device of claim28, further including a nickel oxide layer situated on said secondferromagnetic layer.
 35. A spin-based device including a thin film stackcomprising: i) a first ferromagnetic layer characterized by a firstcoercivity; ii) a second ferromagnetic layer characterized by a secondcoercivity which is substantially greater than said first coercivity;iii) a third layer situated between said first ferromagnetic layer andsaid second ferromagnetic layer and adapted as a low transmissionbarrier to a spin based current; iv) a fourth layer situated in saidthin film stack so as to promote magnetic anisotropy in said secondferromagnetic layer.
 36. The spin based device of claim 35 furtherincluding an oxidation protection layer that also enhances current flowin said first ferromagnetic layer.
 37. The spin based device of claim 35further including a fifth layer located in said thin film stack which isadapted as a second low transmission barrier.
 38. The spin based deviceof claim 35 wherein said low transmission barrier is situated at aninterface of at least one of said first ferromagnetic layer and saidsecond ferromagnetic layer.
 39. The spin-based device of claim 35,wherein at least one of said first ferromagnetic layer and said secondferromagnetic layer comprises a bi-layer of two different materials. 40.The spin-based device of claim 35, wherein the device is characterizedby a transimpedance value which increases as dimensions of said devicedecrease.
 41. The spin-based device of claim 35, wherein said thirdlayer further includes a base layer comprised of at least one of gold,silver, aluminum and/or copper.
 42. The spin based device of claim 35wherein said first coercivity is less than half of said secondcoercivity.
 43. The spin based device of claim 35, wherein said deviceis situated on an insulator.
 44. The spin based device of claim 43,wherein said insulator is situated on a silicon substrate.
 45. The spinbased device of claim 44, further including one or more semiconductingdevices in said silicon substrate.
 46. The spin based device of claim35, wherein said spin based current flows perpendicular to amagnetization plane of said first ferromagnetic layer and said secondferromagnetic layer.
 47. A spin-based device including a thin film stackcomprising: i) a first ferromagnetic layer with a first coercivity and avariable magnetization state that can be altered in response to amagnetic field; ii) a second ferromagnetic layer with a secondcoercivity which is substantially greater than said first coercivity andhaving a fixed magnetization state; iv) a third layer situated in saidthin film stack so as to promote magnetic anisotropy in said secondferromagnetic layer; iii) a fourth layer situated at an interface tosaid first ferromagnetic layer and/or said second ferromagnetic layerand adapted as a low transmission barrier to a spin based current;wherein a value of an impedance in the spin based device for said spinpolarized current flowing through the stack varies in accordance with arelationship between said first changeable magnetization state and saidsecond non-changeable magnetization state.
 48. The spin-based device ofclaim 47, wherein a coupling parameter α=H/I is configured to be between5 and
 20. 49. The spin based device of claim 47 further including afifth layer located in said thin film stack which is adapted as a secondlow transmission barrier.
 50. The spin based device of claim 47, whereinsaid device is situated on an insulator.
 51. The spin based device ofclaim 50, wherein said insulator is situated on a silicon substrate. 52.The spin based device of claim 51, further including one or moresemiconducting devices in said silicon substrate.